Great strides in wireless telecommunications techniques have been made in recent years and this has been accompanied by the implementation and practical use of various wireless communications systems. A mobile radiotelephone system or so-called cellular phone system best typifies such wireless telecommunications systems.
Various wireless telecommunications schemes have been proposed for such mobile radiotelephone systems. For example, FDMA (Frequency Division Multiple Access) is currently being employed in analog mobile radiotelephone systems. According to the FDMA scheme, a single communication channel is allocated to a single radio frequency and communication channels used by respective ones of a plurality of users are set to different radio frequencies on a per-user basis.
TDMA (Time-Division Multiple Access) is currently being employed in digital mobile radiotelephone systems, which have rapidly come into widespread use in recent years. With TDMA, a single radio frequency is divided up into a plurality of time slots and signals are transmitted at the timings of time slots that have been allocated to the local station, whereby three or six communication channels, for example, are formed on one radio frequency. In accordance with this scheme, a plurality of communication channels can be formed on a single radio frequency, as a result of which the capacity of the system can be enlarged over that of the analog scheme.
CDMA (Code-Division Multiple Access) has recently been proposed as a communications scheme for the next-generation of mobile radiotelephone systems. With CDMA, the same radio frequency band is shared simultaneously by a plurality of users. At the time of transmission, transmitted data is multiplied by spreading codes that differ for each user, thereby forming a plurality of channels on the same frequency. Since this scheme makes it possible for the same frequency band to be used by all cells of a cellular system, the system capacity can be increased further over that of the TDMA scheme. For this reason, CDMA is currently the focus of interest in regard to its application to the next-generation of mobile communications.
Since a mobile radiotelephone systems using CDMA is such that common use is made of the same frequency, a situation arises in which a signal transmitted by a certain user acts upon another user as interference. This is an important problem encountered when constructing the communications system. Accordingly, a variety of expedients for reducing interference waves in CDMA communications system have been devised. One is the usage of an adaptive array antenna. An adaptive array antenna increases system capacity by reducing interference waves inflicted upon other users.
An adaptive array antenna is composed of a plurality of antenna elements. When the amplitudes and phases of signals to be transmitted are adjusted and the signals are then supplied to respective ones of the antenna elements, the signals sent from the antenna elements are combined and, as a result, a signal beam of a radio signal is formed in a prescribed direction, as illustrated in FIG. 7. Since a signal S1 is thus capable of being transmitted almost solely in the direction of a mobile station M1, it is possible to prevent the transmitted signal S1 from acting upon a mobile station M2 as interference.
The number of array elements in an adaptive array antenna is decided depending upon the extent to which a signal beam is to be concentrated on a specific user, the extent to which signals are to be isolated and the degree to which mutual interference between users is to be reduced. An adaptive array antenna of this kind is disclosed in detail in IEEE ASSP Magazine, pp. 4-24, April 1988; Barry D. van Veen and Keven Buckley: “Beamforming: A versatile approach to spatial filtering”.
There is demand for various functions other than a voice function, and particularly intense demand for a multicast function, in next-generation mobile radiotelephone systems. A multicast function is a function for transmitting the same information from a base station to a plurality of mobile stations simultaneously. A specific example of a service that relies upon the multicast function is the simultaneous transmission of information (referred to as “multicast data” below) such as traffic information, weather information and stock-market information to a plurality of contract users.
If an attempt is made to implement such a multicast function using a CDMA mobile radiotelephone system, a conceivable approach is to form signal beams S1 and S2 individually in the directions of contract mobile stations M1 and M2 using an adaptive array antenna in a manner similar to that of ordinary individual communication, and transmit the multicast data individually by these signal beams S1, S2, as shown in FIG. 8.
With this method, however, the multicast data is transmitted upon forming the signal beams S1, S2 individually for the mobile stations M1, M2 of the contracting parties. This is inefficient in terms of multicasting. Further, the multicast data is transmitted separately to the mobile stations M1, M2. This also is inefficient in terms of transmission power. Thus, inefficiencies remain in regard to the transmission of multicast data and there seems to be room for improvement.